Topological phases of matter have been demonstrated to exist at the surface of materials with strong spin-orbit coupling, having topologically protected surface states with novel physical properties including relativistic dispersion, spin locked to momentum, and dissipationless spin currents. Measurements based on infrared (IR) light could greatly enhance resolution. Moreover, potenial optoelectronic, spintronic, and photovoltaic TI based devices should be able to operate in this range. The main experimental challenge is obtaining controlled optical access to TI surface states, while avoiding the bulk of the material.
We demonstrate a device, employing the unique advantage of surface plasmon-polaritons (SPPs) to confine rotating electromagnetic (EM) fields to potential TI surface-states on a metal-TI interface. Having high permittivity values, TIs such as Bi2Se3 (e.g. ℜ[ε(@λ=1.5μm)]~26) facilitate strongly confined SPPs that demonstrate significant propagation lengths >10 SPP wavelengths.
To couple the photons to the spin-helical TI surface-states we implemented a plasmonic square cavity. FDTD simulation in Fig. 1 show that under circularly polarized illumination, the phase between the vertical and horizontal in-plane SPP field components in the cavity arranges in a chess-board-like pattern (Fig. 1a,b.). Each square carries predominantly "up" or "down" spin angular momentum (AM). This allows coupling to a different branch of the spin-helical topological state in adjacent squares.
The out-of-plane SPP field component arranges in an array of scalar charge +/-1vortices (Fig. 1d-f.), contributing orbital angular momentum (AM) without substantially affecting the spin AM distribution. This vortex array was experimentally measured by an aperture-less pseudoheterodine NSOM system (Fig. 2a,c,d). The square cavity was engraved by FIB on gold, with few concentric squares to increase the SPP generation efficiency (Fig 2b.).
Finally, we show that switching the handedness of light polarization, flips the spin in a given chess-board square (Fig 1a,d v.s. b,e.) allowing to design spectroscopic experiments controlled by the circular-polarization of excitation light.
spektorg@tx.technion.ac.il